We know that serif and sans serif can perform equally well, if you choose a body text size above 12 pixels. Below 12 pixels serifed typefaces don’t render sharply enough, but on desktop monitors 12 pixels, I personally think that is definitely too small anyway. So my question is: do you think we should ban 11 pixels font-size on any of our design? Serif or sans serif? Should I include that in my functional/specs requirements documents?

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I have always learnt that sans-serif are better for screens if you read a lot and the opposite for paper. Do you have research for your 12 pixel claim. And are you sure it is pixels? Or do you mean points?
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JeroenEijkhofJul 5 '12 at 18:16

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There are too many variables to create an absolute rule such as "fonts must be larger than X in size". How big is the viewport being used (physical dimensions of a pixel)? What font is being rendered? Who are your users? How close are they to the screen? All of those questions would need to be answered to reasonably judge what size a font should be.
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zzzzBovJul 5 '12 at 18:24

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Also, this site uses a 10px font-size for the copyright text. Is it unreasonably small?
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zzzzBovJul 5 '12 at 18:26

@zzzzBov That's OK for a single line of text which needs to be tucked out of the way. I wouldn't want to have deal with a block of it on the iPad.
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Andrew LeachJul 5 '12 at 18:38

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To echo @zzzzBov there are way too many variables that go into the readability of online text than just the particular size spec'ed to universally claim that a particular size should be banned.
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DA01Jul 5 '12 at 18:44

2 Answers
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No

There are too many variables to consider before you can reasonably ban a particular sized font in a design.

That doesn't mean you should start using 8px fonts all over the place. There's an appropriate time and place; a good designer will understand when to use it and when to avoid it.

If you're designing for a billboard, you may have to ban fonts less than 3 meters so that observers can actually read it from a distance. If you're designing for an array of lightbulbs, your font-size might be restricted to 6px because that's all you've got.

Additionally, some fonts become blurrier than others at small font sizes. A font-size of 9px might be reasonable for one font where another can't go any smaller than 13px.

As I noted in the comments on the question, our own ux.stackexchange.com site uses a 10px font in the footer for the copyright notice. The small font is being used on purpose so that it's out of the way. If it were larger, it'd distract from the important navigational elements that are located in the footer. Most users don't care about the copyright notice, and the few that do are always able to zoom in their web browser if they're unable to read the text as-is.

This "pixel" question is not relevant because 11 pixels looks different for each font and legibility is also dependent on screen resolution and many more parameters.

"Serif or sans serif?"

For more on your question on the topic Serif VS. Sans-Serif:

Alex Poole says: This article gives a great introduction to the topic (readability vs legibility / serif vs sans-serif) This has become a must-read on this topic in my mind. That is because of the amount of research it references but also how Poole brings perspective to why some of the research is invalid due to poor methodology.

Some good excerpts:

"Most disappointing however, is that in more than one hundred years of legibility research, researchers have failed to form a concrete body of theoretical knowledge on the part that serifs may play in legibility ( Lund, 1999 ). Nor have they managed to make their work sufficiently known in the typographic community ( Spencer, 1968, p.6 )."

Preference is the same as performance ... or is it?

"What is important to bear in mind is that in almost all legibility studies, reader preference or perceived legibility tends to be inconsistent with user performance ( Lund, 1999 )."

Sans-serif is for the web right?

However, this has not been borne out by recent evidence ( Bernard, 2001 , Boyarski et al., 1998 , Tullis et al., 1995 , De Lange ), that shows no difference in legibility between serif and sans serif font on the web.

Wikipedia says: Studies "conflict" and are "ambiguous", leading to weak arguments for or against serif/san-serif fonts.